Since emulsion resins include water as a solvent and are safe in terms of flammability and toxicity, they have been widely used for adhesives, paints and the like. Generally, the lowest temperature at which an emulsion resin forms a continuous film on drying is called a minimum film forming temperature (MFT). The MFT differs depending on the type, composition or the like of the emulsion resin, but the MFT of generally used emulsion resins is within a range of 10° C. to 70° C. Therefore, problems arose, particularly, when an emulsion is used in winter, such as where the emulsion is dried at a temperature lower than the MFT, no transparent continuous film is formed and whitens, cases where sufficient performance cannot be demonstrated or decorative properties of the film are degraded. Among such resins, the MFT of acrylic emulsion resins is particularly high, and for a large number of emulsions, sufficient adhesive strength cannot be obtained even in summer. For this reason, although treatments such as performing forced heating or ensuring sufficient temperature when the film is formed are conducted, long-term treatment at high temperature is required, and therefore a film forming aid that makes it possible to reduce the MFT and form a coating film over a short period of time is needed.
Generally, a plasticizer or a solvent with a high boiling point is added as the film forming auxiliary agent. Dibutyl phthalate, dimethyl phthalate, and tricresyl phosphate are typically added as the plasticizer, and butyl cellosolve, butyl carbitol, and N-methylpyrrolidone are typically added as the solvent with a high boiling point. In particular, dibutyl phthalate is generally used because of low cost and good stability of the plasticized emulsion, but the film forming ability thereof is not very good and a large amount thereof is needed to obtain an emulsion capable of forming a film at a low temperature. Other drawbacks are that the formed film is plasticized too much, the heat resistance of the film is decreased, and creep easily occurs under load. Further, the use of phthalic acid esters typified by dibutyl phthalate has been avoided since as environmental hormones, they have been indicated as causing reproductive abnormalities.
On the other hand, the drawbacks of solvents with a high boiling point, such as butyl cellosolve, butyl carbitol, and N-methylpyrrolidone, are that they include peculiar odors and reduce the water resistance of the formed films. Although superior to dibutyl phthalate in film forming property, the performance thereof can hardly be termed sufficient. Further, such solvents raise concerns about their adverse effects on humans and are regulated as volatile organic compounds (VOC). The use thereof is thus limited. More specifically, in order to suppress the emission of organic compounds released into the atmosphere, the emission amount of organic compounds has been regulated by the Japanese Air Pollution Control Law, but since compounds with a boiling point of 260° C. or lower have an increased emission amount, it is desirable that organic compounds with a boiling point of 260° C. or higher be used as the film forming auxiliary agents.
A variety of such compounds have been studied as film forming auxiliary agents, and film forming auxiliary agents constituted by specific ester compounds thereamong have been confirmed to have a high boiling point, no odor, and good film forming ability. For example, Patent Document 1 discloses a film forming auxiliary agent for an acrylic emulsion resin, which is constituted by a compound having 2 ester bonds and 1 to 10 ether bonds.